A highly reproducible in vitro assay system has been developed for investigating the biosynthesis and structure of melanin pigment in live chick retinal pigment epithelium tissues. These tissues rapidly utilize extracellular tyrosine for their melanin synthesis but apparently also have sizeable cellular pools of tyrosine. Evidence has been obtained that tyrosinase enzyme activity is the regulation point for overall control of the rate of melanin synthesis. Because of its exceptional inertness, melanin has proven singularly resistant to structure determination by conventional chemical methods. The extent of retention of various radioactively-labeled tyrosine molecules in incorporation experiments with live cells permits interesting deductions about melanin polymer structure and assembly. Studies of pigment epithelium growth and melanotic activity during the middle part of embryonic development (days 6 to 14) illustrate a very interesting regulatory phenomenon. Melanin synthesis and tyrosinase activity remain constant in tissues which are undergoing a diminished rate of growth and rate of overall protein synthesis.